This invention relates to a plasma switch for protecting a radiation-responsive device, such as an infrared sensor from excessively intense laser beams and, more particularly, to the use of a nonlinear crystal for obtaining a harmonic component of incident radiation, the harmonic component lying in an absorptive spectral region of the switch for inducing sufficient plasma density to reflect excessively intense radiation at the fundamental component of the incident radiation.
Radiation sensors and other devices are employed for communication and other purposes with coherent radiation, particularly lasers. Of particular interest, herein, is the transmission and reception of infrared radiation over a predetermined spectral band, such as infrared radiation having wavelengths in the range of 7-12 microns. Generally, at a receiving site such radiation is of a sufficiently low intensity to permit the use of optical focusing elements for directing incoming radiation to a sensor of the radiation. For example, such radiation may be emitted by subject matter in the form of a scene which is to be viewed by an array of radiation sensors or detectors to form an image of the scene. Alternatively, a sensor might be employed to receive data communicated optically from a distant source via a laser beam.
A characteristic of infrared radiation sensors is their operation at low temperatures with incident infrared radiation of relatively low intensity. Infrared radiation sensors are sensitive to temperature. If a beam of radiation incident upon the sensor were sufficiently intense to as to heat the sensor, such heating may well raise the temperature sufficiently to introduce catastrophic failure of the sensor. In low level radiation, the incident radiation does not produce any substantial temperature rise which might interfere with the proper operation of the sensor.
In the use of optical systems employing radiation-responsive devices which might be damaged by excessively intense radiation, there is the inherent danger that an unwanted source of radiation, such as a strong laser, may direct a beam of the radiation towards the receiving optics of the sensor system. Such a situation might arise by an accidental direction of a strong laser towards the sensor system, or might arise from a deliberate attempt as in warfare to damage the sensor.
Thus, there is a problem in that infrared sensors and similar radiation-responsive devices require protection from excessively intense beams of laser radiation. One attempt at solving this problem has been the employment of mechanical shutters which, in response to the detection of excessively intense incident radiation, are activated to close off a path of propagation of the radiation to the sensor, thereby to protect the sensor from the intense radiation. Such mechanical shutters suffer a disadvantage in that the response time for moving the shutter into the radiation path may well be excessively long. As a result, the sensor may suffer severe damage before the incident radiation has been fully blocked from the sensor by the shutter.